For navigation grade nuclear magnetic resonance (“NMR”) gyroscopes, a shielding attenuation factor of less than 1×105 is desired because the uncertainty in the gyroscope axis alignment for a navigation grade gyroscope must be less than about one arc-second. Using a conventional method, external fields aligned along the internal magnetic field vector can be compensated. However, an external magnetic field with a component orthogonal to the spin axis will result in a change in direction of the internal magnetic field vector, which is an axis misalignment. The range of attenuation factor that is required to keep the axis alignment error less than one arc-second depends upon the maximum external magnetic field expected to be encountered and the magnitude of the internal magnetic field. The shielding should be sufficient to keep the orthogonal external field penetrating the shield at less than five parts per million of the applied internal field.
Simple calculations show that at least two layers of magnetic shielding will be required. However, it may be difficult to provide multiple robust shielding layers for very small (down to one cc volume) NMR gyroscopes, such as those used in inertial measurement units (“IMUs”). At this diminutive size, it may be difficult to provide robust shielding, considering the multiple electrical and mechanical connections needed for NMR gyroscope operation.
As a further complicating factor, NMR gyroscopes are generally not used singly, but are commonly used in an orthogonal arrangement of three gyroscopes. This triple-gyroscope configuration is desirable because when the true axis of rotation is not along the spin axis, the NMR gyroscope does not measure the component of rate about the spin axis in a conventional manner. Instead, corrections must be applied based upon the output of all three individual NMR gyroscopes. Therefore, the shielding requirements are complicated because of the necessity of maintaining an operable yet small package of multiple gyroscopes.
An example of an appropriate means for preventing magnetic interference with the NMR gyroscope is a two-layer shielding structure. In this two-layer structure, both the inner and outer shields may be formed of a high magnetic permeability material, which is subjected to a stress annealing process to enhance and maximize its magnetic permeability. Such shields are often made of a ferromagnetic material having high relative magnetic permeability. When the magnetic permeability of a shielding material is high, the magnetic flux paths are substantially diverted through the structure of the shield and do not reach the contents being shielded. Commonly used high permeability materials include alloys, such as Carpenter High Permeability 49® and/or Carpenter HyMu 80® (trademarks of Carpenter Technology Corporation of Reading, Pa.), whose compositions are respectively a 48% nickel-iron alloy and an unoriented 80% nickel-iron-molybdenum alloy.